In addition to periodic incremental signals regarding the relative offset of two objects that are displaceable relative to one another, certain conventional position-measuring devices also supply so-called reference-pulse signals. With the aid of the reference-pulse signals, it is possible to obtain an exact absolute reference regarding the position of the two mutually displaceable objects at individual specified reference positions along the measuring path. The incremental measurement, which typically has a much higher resolution, can be related to the absolute position determined in this manner.
In connection with the generation of reference-pulse signals, a number of possibilities for the placement of reference markings on the measuring graduation scanned by the scanning unit are conventional.
A typical arrangement, for example, places one or a plurality of reference marking(s) laterally adjacent to the scanned incremental graduation track. In this regard, reference is made to European Published Patent Application No. 0 194 611, for example. However, a problem with such an arrangement is that, if undesired tilting about an axis perpendicular to the measuring-graduation plane (a so-called Moiré tilt) occurs between scanning unit and measuring graduation, this will cause a shift in the position of the generated reference-pulse signal relative to the incremental signal. The result is a faulty position determination.
For that reason, arrangements are described, for example, in European Published Patent Application No. 0 303 008, in which two incremental scale-division tracks are situated on both sides adjacent to a reference marking on the measuring graduation. The two incremental scale-graduation tracks are disposed at a mutual offset, the offset amounting to a partial amount of the graduation period, so that their separate scanning via separate scanning devices allows the generation of two 90° phase-shifted incremental signals. While faults with regard to possible Moiré tilting are able to be minimized by such an arrangement, the proposed scanning of the incremental scale-graduation tracks still is not optimal, in particular for high-resolution position measurements. For one, this approach is relatively expensive due to the separate scanning of the incremental scale-graduation tracks. In addition, if one of the two incremental scale-graduation tracks becomes contaminated, faulty measurements may result.
Furthermore, a high-resolution position-measuring device is described in German Published Patent Application No. 197 48 802, in which reference markings in the form of scale-graduation structures which are oppositely chirped in the measuring direction are disposed laterally adjacent to an incremental scale-graduation track. Similar arrangements on the side of the measuring graduation are also described in German Published Patent Application No. 199 21 309 and German Published Patent Application No. 40 21 010.